Multilayer circuit boards (MLBs) typically consist of alternating layers of copper and dielectric material, which are laminated together. There are a variety of MLBs on the market today. Depending on the materials they are made of, they can be rigid (if formed from rigid materials such as glass-reinforced epoxy-resin, BT, CE, PTFE, etc.), flexible (if formed from flexible materials such as polyester or polyimide) or they can be a combination, (i.e., so-called "rigid-flex"). If the alternating layers of copper and dielectric are applied one onto another in a sequential process rather than laminated simultaneously into a single package, they are classified as Sequential Build-Up (SBU) Boards. The integrity of MLB depends upon a reliable bond being formed between the copper and dielectric. In conventional MLB manufacturing processes, copper layers are treated using a hydroxide/chlorite based treatment solution. This so-called "Black Oxide Process" provides an enhanced copper-to-dielectric bond. Examples and details of the Black Oxide Process are set forth in, for example, U.S. Pat. Nos. 4,358,479; 4,409,037; 4,512,818; 4,844,981 and 4,969,958. However, the Black Oxide Process suffers from various limitations including the following:
1. The process can be carried out only in an aggressive strongly alkaline solution at about 160-170.degree. F.
2. The processing time is long, thereby creating bottlenecks in the manufacturing sequence.
3. The Black Oxide coating readily dissolves in acids, which makes it susceptible to acid attack in subsequent processing steps. This frequently results in an undesirable phenomenon commonly referred to as "pink ring" and sometimes can even result in "wedge voids" and ultimately delamination between the copper and adjacent dielectric layer.
4. The initially high bond strength between the Black Oxide coated copper and the dielectric material usually deteriorates rapidly as the MLB passes through subsequent thermal excursions during the fabrication and assembly processes.
A reliable copper-to-dielectric bond is especially important for flex and SBU circuit boards. Polyimides, which are used for flexible boards, and photo-imageable dielectrics used for SBU circuit boards, are known to exhibit lower copper-to-dielectric bond strength than conventional epoxy-glass based rigid circuit boards.
The Black Oxide coating comprises a mixture of cupric and cuprous oxides. This coating exhibits poor chemical resistance on the basis that it is predominantly formed of cupric oxide. Unlike cupric oxide (black) which is easily attacked by acids, cuprous oxide (red to brown) is relatively chemically inert.
In an attempt to eliminate "pink ring" and to prevent deterioration of the bond strength, two types of post-treatments for the Black Oxide Process have been developed. The first type is based on a reduction process as disclosed in, for example, U.S. Pat. Nos. 4,642,161; 4,902,551; 5,006,200; 5,076,864; 5,147,492; 5,382,333; 5,492,595; 5,556,532 and 5,721,014. The second Black Oxide post-treatment type is based on partial dissolution of the oxide coating as described in, for example, U.S. Pat. Nos. 4,717,439; 4,775,444; 5,106,454; 5,261,154; 5,289,630 and 5,501,350. Both of these techniques achieve similar results, although they do so through different chemical mechanisms. The first mechanism is a reduction process, which partially converts the cupric oxide into a mixture of cuprous oxide and metallic copper. The second mechanism is a dissolution process, which selectively dissolves cupric oxide, thereby leaving cuprous oxide on the surface. Both processes convert Black Oxide (predominantly cupric) into a surface rich in cuprous oxide. Unfortunately, however, although Black Oxide reduction and dissolution post-treatments do eliminate "pink ring" and improve the acid resistance of the coating to some extent, they do not solve all of the above mentioned problems associated with the Black Oxide performance.
Furthermore, reduced Black Oxide has been found to suffer from a rapid re-oxidation of the surface during baking steps, which are used in MLB manufacturing processes in order to remove absorbed moisture from the dielectric prior to the lamination steps. The re-oxidation of the surface manifests itself by a noticeable darkening of the coating after the baking step. The darker color of the coating indicates the reappearance of cupric oxide, which once again makes the coating more susceptible to chemical attack in subsequent processing steps.
Partially dissolved Black Oxide is susceptible to the formation of a powdery, poorly adherent layer of cuprous oxide which forms on the surface due to undercutting action of the chemical bath used for the dissolution. This powdery portion of the coating prevents the formation of a strong copper-to-dielectric bond. In addition, it makes it impossible to use this process in a horizontal conveyorized mode due to the formation of roller marks, which can occur in the powdery surface.
Recently a new alternative to the Black Oxide Process described above has been introduced. Instead of forming cupric oxide, it produces an organometallic conversion coating (OMCC) on the copper surface. In general, the purpose of organometallic conversion coatings is to form a barrier layer on the underlying metal surface, thereby protecting it from oxidation and corrosion, and/or increasing its bond to the dielectric material. OMCCs are the product of a chemical reaction between metal ions (in single or multiple valency states) and organic compounds. As such, they are readily distinguished from inorganic Black Oxide. The OMCC comprises a film of metal ions tied into a complex or otherwise bonded with one or more organic materials. Such processes for fortning an OMCC are disclosed in U.S. Pat. Nos. 5,800,859 and 5,869,130.
The organometallic conversion coating that forms during this process is much more acid resistant than Black Oxide coatings. It is not, however, completely impervious to chemical attack since it still contains a portion of a cupric-based organometallic compound in its composition.
The mechanism of the "wedge void" formation discussed below will illustrate the significance of the inventive post-treatment step.
After lamination the MLB panels are drilled. During the drilling operation, vibration of the drill bit can create microfractures in the copper/dielectric interface. Solvent conditioners then can get absorbed into the microgaps between the copper and the dielectric. Subsequent treatment with permanganate solution will oxidize, i.e. remove, this already swelled resin creating a "wedge". Microetch solution will attack the copper and widen the gap. Pre-dip and activator solutions typically are fairly concentrated hydrochloric acid based solutions, which will further remove the copper treatment layer. If electroless copper fails to completely seal the "wedge", the chemical attack will continue when the article is immersed in acid cleaner, microetch solution, acid dip and acid copper. When the "wedge" is relatively small, acid copper will completely fill the gap resulting in typical "pink ring" appearance. If the gap is large enough, it may bridge only partially or may not bridge at all leading to a "wedge void" defect. In the most severe cases, the result is delamination.
The above notwithstanding, a need still exists for an improved method of enhancing the copper to dielectric bond in MLBs, while at the same time providing enhanced chemical resistance to the resulting article.